Summary: Most injuries to the brain or spinal cord spare some connections between the areas of brain that initiate movement and the areas of spinal cord that produce movement. A pivotal question for recovery of movement is the degree to which spared connections can compensate for injured ones. We will study the adaptation of spared motor pathways to injury of the corticospinal tract (CST), the principal pathway for voluntary movement in humans. The CST, which connects the motor cortex to the spinal cord, controls fine hand movements and modulates spinal cord reflexes. We will cut the CST emanating from one hemisphere and test the ability of spared circuits to take over the lost function. Specifically, we will examine two circuits: 1) the uninjured half of the CST through its sparse ipsilateral projections, and 2) a bypass circuit on the injured side from cortex to red nucleus to spinal cord. We will injure rats soon after birth, a time when these connections are plastic and the opportunity for compensation is highest. We then measure the response of these spared systems using two novel techniques-retrograde transsynaptic tracing using pseudorabies virus, and stereological quantification of axonal connections. Thus, we will determine which spared system sprouts greater connections in response to injury. In adult rats with neonatal injury to one half of the CST, we will test the functional limits of pathway compensation. We will use both novel and proven tests of functions that depend critically on the CST: reaching to grasp, walking over a ladder, food manipulation, and control of a spinal cord reflex. We predict that there will be incomplete recovery of the specialized motor skills. For functions that do recover, we will temporarily inactivate each of the two spared pathways, by infusing an inhibitor of neural activity, to test their contribution to recovery. We expect that the recovered functions will be lost transiently in the pathway that shows the greatest amount of injury-induced plasticity. Finally, we will selectively activate the most adaptive circuit to try to improve upon endogenous recovery. Using chronic stimulation through an implanted electrode, we intend to harness activity-dependent plasticity to strengthen motor connections. We predict that these targeted manipulations will create a more adaptive pattern of brain- spinal cord connections, as measured by tracing and stimulation techniques, and help to restore function, based on the aforementioned motor tasks. Many human infants, especially those born prematurely, sustain injury to the CST. This often results in paralysis and spasticity on one side of the body. Activity, in the form of physical therapy and non-invasive brain stimulation can be used to alter connections in humans. These studies will help to determine where activity should be applied in order to strengthen the circuits that mediate spontaneous recovery. Thus, we use anatomically precise injury, tracing, inactivation, and stimulation to determine the circuit-level logic for repair of the motor systems. This could improve our ability to restore function in people with early brain injury.